Refining lead



Pateiited 19, i940 ""UNITED TATES PATENT OFFICE REFINING LEAD Harvey N. Gilbert, Niagara Falls, N. Y., asslgnor to E. I. du-Pont de Nemours & Company, Wil- I mington, Del., a corporation of Delaware No Drawing. Application January 31, 1939, Serial No. 253,861

7 Claims.

This invention relates to the purification of lead or lead alloys and more particularly to the removal of bismuth from lead or lead alloys.

'Heretofore lead or lead alloys containing bismuth hav been refined to remove the bismuth contained therein by treating the molten lead or alloy with an alkaline earth metal such as calcium or magnesium, with or without the addition of other refining agents. An improvement in this art consists in adding potassium together with the alkaline earth metal. For example, as shown by Patent 2,133,327, when both potassium and magnesium are added to the bismuth-containing lead, a better separation of bismuth is obtained than when magnesium alone is used. In carrying out this method, metallic potassium and metallic magnesium are added to the molten lead bath. When the reaction with the bismuth is complete, the bath is cooled to a temperature slightly above its solidification point and the resulting dross which forms on the surface is removed.

A difliculty inherent in such processes is that metallic potassium, because of its highly reactive nature, is difficult and hazardous to handle. Metallic potassium has the property of reacting with the oxygen of the air at ordinary temperatures to form on the surface of the metal a film of a highly reactive peroxide K204. Because of the highly oxidizing nature of this peroxide it rapidly attacks the underlying metal, generating heat, sometimes with explosive violence. This highly reactive property of potassium frequently limits the use of this metal in metallurgical operations, where it is difficult and unduly expensive to exercise the care required to prevent oxidation by the air.

An object of the present invention is to provide an improved method for the removal of bismuth from lead or lead alloys. A further object is to provide an improved process for removing bismuth from lead which does not require the handling of hazardous materials. Further objects will be apparent from the following description of my invention.

These objects may be attained in accordance with the present invention by treating a molten bath of lead or lead alloy containing bismuth with magnesium or other alkaline earth metal, potassium hydroxide and sodium. I have discovered that in this manner the amount of bismuth removed is substantially the same as when the bath is treated with potassium and magnesium and there are substantially no hazards due to air oxidation.

In one method of practicing my invention I prepare a bath of lead to be refined by melting it and holding it at a temperature of MiG-500 0. Sodium and magnesium in the required amounts are added and caused to dissolve in the 5 molten lead. A convenient method of adding the sodium is to place solid sodium in an open container provided with a handle and hold the container in an inverted position beneath the surface of the bath until the sodium has become 10 dissolved in the lead. Another convenient method is to stir the lead bath to create a vortex and to add bricks of sodium or molten sodium to the vortex, whereby the sodium is drawn beneath the surface of the bath. If desired the lead 18 alloys of sodium and magnesium may be added instead of the pure metals. Various other known methods of adding sodium to molten metals such as molten lead, which are well known to those skilled in lead refining, may be utilized.-

After the sodium and magnesium have become dissolved in the lead bath, a quantity of potassium hydroxide is added. If the commercial potassium hydroxide is used, there is some tendency for spattering to occur because of the water contained in the commercial product. Accordingly, it is preferable .to use substantially anhydrous potassium hydroxide. When the anhydrous material is not available, I prefer to dehydrate the potassium hydroxide by melting it in a separate container and add sufficient sodium to react with the water present, thus forming a mixture consisting mainly of potassium hydroxide together with sodium hydroxide equivalent to the amount of water originally present. If desired, this hydroxide mixture then may be cooled'and the solid material added to the lead bath. It is preferable in most cases, however, to add the molten hydroxide to the lead bath.

In order that the potassium hydroxide may react with the bismuth and other ingredients to cause the removal of the bismuth, it is preferable to rapidly stir the lead bath, after the addition of the hydroxide, or both during and after its addition, until the reaction with the bismuth is complete. A preferred method, for example, is to stir the lead bath so as to form a vortex and to pour the moltenpotassium hydroxide into the vortex. Preferably, the stirring is continued as vigorously as possibleuntil the reaction with the bismuth is complete. Th lead bath then is cooled, for example, to a temperature of 330-350 C., agitation is stopped and the resulting dross which floats on the surface of the bath is skimmed oil. The dross will contain the bismuth together u I with magnesium, sodium and the hydroxides of potassium and sodium. If a single treatment does not completely remove the bismuth, or remove it to the desired extent, the process may be re- The quantities of the reagents will depend upon I the amount of bismuth in the bath or the amount of bismuth to be removed. Generally it is prefand to then cool to 330 to 350 erable to add about equal amounts of the magnesium, potassium hydroxide and sodium. The

amount of magnesium-should be at least equal to the amount of bismuth desired to be removed and preferably should be in excess of that amount, for example three to four times as much. While it is generally preferable to add the sodium, magnesium and potassium hydroxide in about equal amounts by weight, these proportions may be varied considerably without departing from my invention. A large excess of any one of the three reagents may be used without ill effect except that such large excess may result in unnecessary waste.

The temperature of the molten lead bath should not be above about 600 C. to prevent volatilization of the metals and corrosion of the equipment. The temperature should be sufiiciently high to keep the potassium hydroxide in a molten state. For pure potassium hydroxide, the minimum temperature is about 380 C., but this may be lower if lower melting mixtures, such as a mixture of the sodium and potassium hydroxides are used. I prefer to carry out the reaction at 400 to 600 C., C. before drossing, to obtain maximum bismuth remov The lead remaining after the dross has been removed which has been freed of bismuth or is of reduced bismuth content usually will contain excess sodium and magnesium dissolved therein. These may be removed, if desired, by known means, for example, by blowing with steam to form the oxides or by chlorination,

In an alternative method of practicing my invention, I may first add the potassium hydroxide and sodium to the bath, preferably stirring well until chemical equilibrium is attained. The magnesium then is added to the lead bath and the bath thereafter is cooled and dressed as described above.

Various modifications of the process may be made without departing from the spirit and scope of my invention, as will be apparent to those skilled in similar metal refining operations. For example, it is notnecessary that the potassium hydroxide added to the melt be pure. It may contain small amounts of other potassium compounds or for example sodium compounds such as caustic soda or sodium salts. For example, a low-melting mixture of potassium and sodium hydroxides may be used. Generally, however, there is no advantage in using such mixtures and the pure anhydrouspotassium hydroxide is preferred, since in such mixtures the desired reactive material is the potassium hydroxide present. Also it is not essential that the bath be stirred, since my method will result in bismuth removal without bath agitation if the potassium hydroxide is permitted to remain in contact with the bath for a sumciently long time. It is preferable, however, to stir the bath as thoroughly as possible, so as to obtain maximum contact between the potassium hydroxide and the lead bath. The order in which the ingredients are added to the lead bath is immaterial; but all three ingredients must be added before removing thebismuth-containing dross. If desired, a layer of molten potassium hydroxide may be placed on the bath before adding the sodium or the magnesium and the sodium. This is sometimes preferred, since the layer of hydroxide, if sufficiently thick, may prevent oxidation of particles of sodium which rise to the surface of the bath before becoming dissolved therein.

By meansof my process it is possible to readily and efficiently remove bismuth from lead or lead alloys without handling hazardous materials such as metallic potassium. The metallic sodium which is used in my process is known to be a reactive material which readily oxidizes in the air. However, it does not possess the hazards of handling which are involved in the use of metallic potassium. Sodium has been used for a. considerable time for refining lead and lead alloys and the safe methods of handling this metal are well known to those skilled in refining lead and similar materials.

1. A lead refining process which comprises adding an alkaline earth metal, potassium hydroxide and sodium to molten lead containing bismuth and thereafter removing the resulting dross.

2. A lead refining process which comprises adding magnesium, potassium hydroxide and sodium to a bath of molten lead containing bismuth and thereafter removing the resulting dross.

3. A lead refining process which comprises adding magnesium, sodium and a liquid melt com prising potassium hydroxide to molten lead containing bismuth at a temperature not higher than about 600 C., stirring said molten lead, and thereafter cooling said molten lead to a temperature near the solidification point thereof and removing the resulting dross.

4. A lead refining process which comprises adding magnesium, sodium and a liquidmelt comprising potassium hydroxide to molten lead containing bismuth at a temperature of about 400 to 600 C., and thereafter cooling said molten lead to a temperature near the solidification point thereof and removing the resulting dross.

5. A lead refining process which comprises adding magnesium, sodium and molten potassium hydroxide to molten lead containing bismuth at a temperature of about 400 to 600 C., stirring said molten lead, and thereafter cooling said molten lead to a temperature of 330 to 350 C. and removing the resulting dross.

6. A process for removing bismuth from impure lead which comprises melting said lead, adding potassium hydroxide and sodium to the molten lead bath, stirring the bath until equilibrium has been reached, then adding magnesium and thereafter removing the resulting dross.

7. The process according to claim 2, in which at least one of the ingredients, magnesium and sodium, is added in the form of a lead alloy.

HARVEY N. GILBERT. 

